1. Field of the Invention
The present invention relates to a focus detection apparatus which performs focus detection by a phase difference method using an image pickup element including a plurality of photoelectric conversion portions sharing one microlens.
2. Description of the Related Art
A focus detection method is well known in the related art, in which taking an image and focus detection by a phase difference method are performed using a solid-state image pickup element including a plurality of photoelectric conversion portions sharing one microlens. Japanese Patent Laid-open No. 2001-83407 discloses a configuration in which two photoelectric conversion portions divided with respect to one microlens obtain pupil-divided image signals to perform a focus detection processing by the phase difference method. In addition, Japanese Patent Laid-open No. 2001-83407 discloses a configuration in which a value obtained by adding the outputs of two photoelectric conversion portions corresponding to the same microlens is handled as an output of a pixel to obtain an image signal as an image pickup signal.
Japanese Patent Laid-open No. 2007-133087 discloses a configuration in which image signals from a plurality of photoelectric conversion portions sharing one microlens are selectively read out as an A-image or a B-image to perform a correlation calculation. Japanese Patent No. 4691930 discloses a configuration in which some of a plurality of divided photoelectric conversion portions are read out through a non-destructive readout and then a synthesis component value of each photoelectric conversion portion is read out. Then, the number of readout pixels is reduced by estimating other divided photoelectric conversion portion based on a difference between the synthesis component value and a pixel value of a part of the photoelectric conversion portions.
Since an output value of the image signal as the image pickup signal is generated by using an addition value of the plurality of divided photoelectric conversion portions in the configuration as disclosed in Japanese Patent Laid-open No. 2001-83407 and Japanese Patent Laid-open No. 2007-133087, an upper limit value of a light amount allowed to be photo-electrically converted in each photoelectric conversion portion is smaller compared with that of the configuration in which the photoelectric conversion portion is not divided. That is, when light having intensity which exceeds a saturation level is exposed on only one of the photoelectric conversion portions, a charge for the saturation are not reflected in the image signal as the image pickup signal, and therefore the image signal as the image pickup signal is not correctly generated.
In addition, it is considered to calculate a correlation waveform for each of a plurality of ranging regions in which the change of correlation value in a column direction is obtained from a signal of each pupil-divided pixel of each pixel when performing the focus detection by the phase difference method using the image pickup element. In this case, the plurality of correlation waveforms calculated from the plural ranging regions are added to be able to calculate a shift amount of a focus by obtaining an image shift amount based on the added correlation waveform.
However, when the correlation waveform is calculated in a region including saturated pixels at the time of performing the focus detection by the phase difference method using the image pickup element, the image shift amount obtained based on the correlation waveform becomes zero and an erroneous focus shift amount is calculated, and thus, focus detection accuracy is deteriorated.